Reduction is performed by simmering or boiling a liquid, such as a stock, fruit or vegetable juice, wine, vinegar or sauce, until the desired concentration is reached by evaporation. This is done without a lid, enabling the vapor to escape from the mixture. Different components of the liquid will evaporate at slightly different temperatures, and the goal of reduction is to drive away those with lowest points of evaporation.[citation needed]
While reduction does concentrate the flavors left in the pan, reducing too much will decrease the amount of all liquid in the sauce, leaving a sticky, burnt coating on the pan.[2]
Sauces, ranging from basic brown saucetobéchamel sauce and even tomato sauce, are simmered for long periods (from 1 to 10 hours) but not boiled. Simmering not only develops the maximum possible flavor, but also allows impurities to collect at the top and be skimmed off periodically as the sauce cooks. Boiling would diffuse the impurities into the liquid and result in a bitter taste and unclear stock.[3] Broths are also simmered rather than boiled, and for the same reasons.[3]: 304–5
